1. Field of the Invention
The present invention relates to a projector that is equipped with a shift projection lens and an imaging unit and functions to project an image on a screen or another projection object and photograph the projected image with the imaging unit.
2. Description of the Related Art
Various projectors equipped with an imaging unit including a CCD (Charge Coupled Device) camera have been proposed to photograph a selected image projected on a screen or another projection object with the imaging unit and to adjust the zoom and the focus and correct a trapezoidal distortion of the projected image (keystone correction) based on the photographed image.
One of such projectors is disclosed in Japanese Patent Laid-Open Gazette No. 2004-312690. This prior art projector analyzes a photographed image, identifies the position of maximum brightness in the photographed image, and makes keystone correction according to the identified position of maximum brightness.
The projector making keystone correction based on the identified position of maximum brightness may have a projection lens that is movable in a direction substantially perpendicular to the optical axis (hereafter called ‘shift lens’). Even when the projector is set at a position other than in front of the projection object (for example, below the projection object) not to block the user's view, a positional shift of the shift lens enables projection of an image at a desired position on the projection object.
In the projector equipped with the shift lens, the position of maximum brightness in the photographed image is required for keystone correction or other image processing operations. One typical method identifies the position of maximum brightness in the photographed image, based on pixel values of pixels on a specified reference line included in the photographed image. This conventional method, however, has some drawbacks due to the specification of the reference line. In the description below, the brightness is expressed by the pixel value.
FIGS. 5(A) through 5(C) show a projection state of a conventional projector PJ without a lens shift, a photographed image taken in this projection state, and a luminance distribution of the photographed image.
The side view of FIG. 5(A) shows a certain projection state of the conventional projector PJ, FIG. 5(B) shows a photographed image taken in the projection state of FIG. 5(A), and the graph of FIG. 5(C) shows a distribution of luminance values of respective pixels on a horizontal line L1 in the photographed image of FIG. 5(B). The horizontal line L1 passes through an intersection between an optical axis of an optical system in the projector PJ and a screen Sc (hereafter this intersection is referred to as the ‘optical axis point’).
As shown in FIG. 5(A), the conventional projector PJ has a shift lens 10 and an imaging structure CA including a CCD camera. In the projection state of FIG. 5(A), the shift lens 10 has no shift.
In the projection state of FIG. 5(A), the projector PJ is inclined to the screen Sc to have an elevation angle. In this state of elevation projection, the projected image has a trapezoidal distortion.
The projector PJ projects an adjustment pattern image G for keystone correction on the screen Sc and photographs an image including the projected adjustment pattern image G with the imaging structure CA. The subsequent keystone correction is made, based on the identified position of maximum luminance in the photographed image.
The imaging structure CA of the projector PJ is arranged to locate the projected adjustment pattern image G on the center of the photographed image in the projection state without a lens shift shown in FIG. 5(A). Namely the projected adjustment pattern image G is located on the center of the photographed image as shown in FIG. 5(B).
In this projection state without a lens shift, the distribution of luminance values of respective pixels on the horizontal line L1 passing through the optical axis point P has a distinct peak with its maximum at the optical axis point P as shown in FIG. 5(C). The shorter distance between the screen Sc and the projector PJ in the coverage of the projection light of the projector PJ causes the higher luminance in the photographed image. Since the projector PJ has no inclination in the horizontal direction in the state of FIG. 5(A), the luminance distribution on the horizontal line L1 has a peak at the position of the optical axis point P.
The projector PJ compares the luminance values of the respective pixels on the horizontal line L1 and identifies the position of maximum luminance as a peak position in the photographed image.
A specified horizontal line referred to for identification of the peak position in the photographed image, for example, the horizontal line L1, is hereafter referred to as the ‘peak position reference line’.
The conventional procedure sets a certain rate h1/H of a height ‘h1’ of the optical axis point P to a height H of the projected adjustment pattern image G in the projection state of FIG. 5(A), and specifies a horizontal line having the height of the certain rate h1/H to the height of a projected image as the peak position reference line.
In order to prevent the projector PJ from blocking the user's view, the user shifts the shift lens 10 upward and moves the position of the projected image further upward.
FIGS. 6(A) through 6(C) show a projection state of the conventional projector PJ with a lens shift, a photographed image taken in this projection state, and a luminance distribution of the photographed image.
The side view of FIG. 6(A) shows a certain projection state of the conventional projector PJ, FIG. 6(B) shows a photographed image taken in the projection state of FIG. 6(A), and the graph of FIG. 6(C) shows a distribution of luminance values of respective pixels on a peak position reference line in the photographed image of FIG. 6(B). The position and the projection area of the shift lens 10 shown by the solid line in FIG. 5(A) are shown by the broken line in FIG. 6(A). In FIG. 6, the same symbols as those of FIG. 5 have the same meanings and are not specifically described here.
An upward shift of the shift lens 10 causes the projector PJ to project the adjustment pattern image G at the upper position on the screen Sc as shown in FIG. 6(A). The projected adjustment pattern image G included in the photographed image of FIG. 6(B) accordingly has a further upward positional shift from the projected adjustment pattern image G in the photographed image of FIG. 5(B). The lens shift causes a trapezoidal distortion of the projected adjustment pattern image G included in the photographed image.
The lens shift does not change the position of the optical axis point P on the screen Sc as shown in FIG. 6(A), so that the relative position of the optical axis point P in the photographed image of FIG. 6(B) is not changed but is fixed. The relative position of the optical axis point P to the projected adjustment pattern image G (hereafter referred to as the ‘relative peak position’) in the photographed image of FIG. 6(B) thus moves downward from the relative peak position in the photographed image of FIG. 5(B).
In the photographed image of FIG. 6(B), the peak position reference line having the height of the certain rate h1/H (see FIG. 5(B)) is a horizontal line L2 having a height h1 from the bottom side of the projected adjustment pattern image G. A point Q in the photographed image of FIG. 6(B) represents a center point of the horizontal line L2.
The relative peak position moves downward as mentioned above, so that the horizontal line L2 or the peak position reference line does not pass through the optical axis point P.
The distribution of luminance values of respective pixels on the horizontal line L2 as the peak position reference line in this projection state has a gentler peak with a relatively indistinct peak position as shown in FIG. 6(C), compared with the luminance distribution of FIG. 5(C).
The distance between the projector PJ and the center point Q is longer than the distance between the projector PJ and the optical axis point P as shown in FIG. 6(A). The luminance value of the center point Q (corresponding to the peak position of the horizontal line L2) is accordingly lower than the luminance value of the optical axis point P (corresponding to the peak position of the horizontal line L1 shown in FIG. 5(B)).
As described above, the lens shift in the prior art projector may give only an indistinct peak position or maximum luminance position in the photographed image. This may result in inaccurate identification of the peak position in the photographed image and accordingly inadequate and inaccurate keystone correction and other image processing operations.